HOW DO HORSES MOVE? 107 
3. Bones.—Certain bones, such as those of the head, 
the vertebra, and the ribs, are useful in protecting the 
organs of the body. For the most part, however, bones 
are a series of crowbars. When the horse moves, these 
bars or levers are operated by the muscles. In no other 
way cana horse move. If any of the movable bones are 
examined, it will be found that they are not smooth bars, 
but that they have many irregularities of surface. The 
projections and depressions in the bones are for the at- 
tachment of muscles. When a muscle is attached to a 
small projection, this projection be- 
comes the small arm of a lever. | x 
4. Lever—When any object does 
what a crowbar does, it is called a Lever of the First Class 
lever. The point where the small 
stone touched the crowbar is called 
the fulcrum. There are three kinds 
of levers, all of which the horse uses. 
They are shown in the diagrams. In 
levers of the first class the fulcrum | 
> 
EY 
a} > <9 
F 
A 
Lever of the Second Clase. 
(F) is between the power (P) and 
the resistance (R). In the second 
class, the resistance is between the sobia veine Shea bias, 
fulcrum and the power, while in the Kinps oF LEVERS 
third class the power is between the 
fulcrum and the resistance. It will be seen that a lever 
of the third class’: must always be a lever of speed, be- 
cause the power arm is always shorter than the resistance 
arm. A lever of the second class must always be a lever 
of power, because the resistance arm is shorter than the 
power arm. A lever of the first class, however, may be 
either a lever of speed or a lever of power, depending 
upon whether the resistance arm or the power arm is the 
F 
A 
<— 
